Bale, 31, has largely been restricted to appearances in the Europa League and domestic cups since making a high-profile return to Spurs on loan in September.
The Welshman has started just once in the Premier League and was left on the bench by Mourinho when Spurs needed late inspiration in Wednesday's 1-1 draw with Fulham.
Bale's season-long loan from Real Madrid is thought to include the option of a second year, which would take him to end of his contract in Spain, but his difficulties have led to speculation that Spurs are already preparing to end his stay at the end of the current campaign.
But Mourinho said: “There was not even a second of discussion about that. Gareth is a player on loan until the end of the season. Not one second of discussion. We are speaking about, of course, Gareth himself, Real Madrid and Tottenham, but I promise you, not one second of discussion.”
"He arrived injured," added Mourinho. "And he was injured before, I believe, the first month. And then, step by step, Europa League matches were helping him to get his condition.
"Europa League group phase is a different level than the Premier League. The knock-out is a different story. But the group phase is a different intensity, a different rhythm, a different quality. So in the Premier League, he didn’t play many minutes.
"Then another small injury at Stoke where he was having a good first half. Not an easy process. Everybody knew that in the past couple of seasons, in Madrid, they were not easy for him. So let’s go, step by step, and try to get the best out of him."
Mourinho side will hope to get their stuttering season back on track at bottom-club Sheffield United, who earned their long-awaited first win of the campaign over Newcastle on Tuesday.
"They're not the worst team in the Premier League at all," Mourinho warned.
"They're a much, much, much better team than what the table says, you analyse their matches and many times the opponent was not better than them. Many times it was one of the small big details that decides the points, decides the score, decides the table.
"Many times they lose 1-0, they draw 1-1, they are a very good team. If in our mind we're going to play the worst team in the Premier League we're going to be in big trouble."
"I focus on two matches we have to play against them and I'm totally convinced we have to play well to win those matches against them."
Sheffield United are among the clubs to draw scrutiny for failing to adhere to the Premier League's new coronavirus protocols, which include instructions for players to avoid celebrating in groups.
"My advice is maybe guy who scores express the emotion of the team by himself, alone," said Mourinho. "Maybe he can express all the emotions of the team and then of course share a walk with mates and happiness. And we try to make things the best we can."
Mourinho is still without Giovani Lo Celso but expects to welcome back Steven Bergwijn after illness.